Vehicle traffic deteriorates a road over time, especially secondary and off-highway trails. On public paved and well-maintained road networks, this deterioration happens very slowly and may be noticeable only after several years. Changes in road roughness are indicators of maintenance requirements. In comparison, durability test courses used for wheeled and tracked vehicle evaluations can change much more rapidly and, depending on the weather and volume of traffic, can have significant changes daily. As the road changes, so does the input to the vehicle. Changes in road roughness may tend to over-test or under-test the vehicle. It is, therefore, necessary that repeatable methods of measuring the profile of the road being used in the test be developed. Further, it is desirable in the testing of vehicles to accelerate the effects of the testing in a controlled manner such that the time required for accomplishing a specified test length can be shortened.
A number of different road profile measurement systems have been in use worldwide. These systems can be classified into two groups: (1) systems that measure the profile directly, such as the surface dynamics profilometer or the rod and level method, and (2) systems which measure vehicle cumulative response to road roughness. The present invention fits into the first category; however, it is unique to that category. Whereas the majority of prior profilometers used pure motion measurements to calculate the terrain, the present invention uses force and motion measurements made at each axle-to-hub interface to calculate the terrain roughness.
The most straightforward method for measuring road profiles is with a surveyor's rod and level. The accuracy with which the profile can be determined is limited only by the resolution of the level instrument and the care taken by the operator and by the interval along the road at which measurements are taken. This method provides the accuracy required but is very tedious and time consuming. The present invention resolves this problem by providing accurate data at a high rate thereby enabling the practical use of road profiling.
An improvement on the rod and level technique was provided by the TRRL Beam, a device developed by the Overseas Unit of the Transport and Road Research Laboratory (TRRL) in England. This unit consisted of a box beam about 10 feet in length which was moved down the road such that a new starting position matched the old end position. The beam was supported by tripods at each end, along which a carriage rolled. The carriage included a vertical member with a pneumatic tire on the lower end which contacted the road and as the carriage was rolled along the beam, the distance from the beam to the road was continuously recorded. This method increased the speed at which measurements could be taken and reduced human error. However, it was still time consuming and prone to accumulation of error each time the apparatus was moved.
Another static method of the prior art for acquiring road profile information was a device supported on two legs spaced one foot apart and having a handle extending upward. The device automatically recorded the inclination angle between the two legs thereby giving a measure of the vertical displacement of the road surface as the device was "walked" along the surface of the road, pivoting first on one leg and then on the other. Again, this device suffered from having to make many independent measurements which lead to an accumulation of error and the method was also time consuming.
The present invention resolves the problems of these static methods by providing accurate data at a high rate while eliminating the accumulation of error.
High-speed profiling systems were developed about 1965 to increase the rate of acquiring road profile information. Most of the systems in use prior to the present invention were based on work done by General Motors Research Laboratory which was described in U.S. Pat. No. 3,266,302 to Spangler and Kelly. Further developments of that basic work by Spangler resulted in U.S. Pat. Nos. 4,422,322, and 4,741,207. These systems were comprised of a vehicle which was instrumented with a vertical accelerometer and a height sensor. The accelerometer measured the vertical motion of the vehicle body and the height sensor measured the distance from the vehicle body to the roadway. Used together, these measurements allowed the road profile to be computed. By compensating for vehicle motions, the resulting profile was related to an inertial reference system.
A variety of height sensors used in the implementation of these systems included mechanical follower wheels, optical sensors and ultrasonic sensors. Each of these prior methods exhibited limitations in profiling rough pavement or off-road terrain. The mechanical follower was limited by its dynamics in that it was required to remain in contact with the road surface at all times. Otherwise, the produced profile represented its path through the air rather than the surface of the road. This resulted in a limit on the speed of the measuring vehicle. Additionally, the mechanical follower was limited to relatively smooth surfaces.
Optical height sensors provided some improvement over the mechanical follower in that the dynamic interaction of the sensor with the road surface was eliminated. The implementation of the optical system usually incorporated a laser light source to beam a bright spot vertically downward on the road surface. A photodetector mounted to the side detected the spot at an angular position relative to the vehicle and variations in that angular relationship were converted to height variations. Since a monochromatic light source was used, filters were used in the photodetector to reduce its sensitivity to extraneous sources of light. One of the significant problems which arose with the optical sensor was a loss of signal when the spot illuminated a hole in the road surface and the photodetector could no longer "see" the spot. This represented a serious limitation to the roughness of the road which could be measured, thereby limiting the technique to relatively smooth, paved road surfaces.
Ultrasound systems can measure height by measuring the time required for a sound pulse to travel to the road surface, be reflected and travel back to the detector. However, a number of problems are presented with this method. The effects of wind and changes in air pressure must be compensated for as they effect the speed of the sound pulse or the path length which the pulse follows. In addition, the condition of the surface of the road effects the ability of the ultrasound detector to receive an adequate signal. If the road surface is too uneven, the sound pulse is scattered and an adequate reflection is not detected. This limits the use of ultrasound height detectors to smooth, paved surfaces. These systems are also sensitive to moisture requiring that the road surface be dry.
All of the aforesaid prior art systems have a common difficulty in measuring the profile of a road surface which is not smoothly paved. The present invention overcomes that limitation by eliminating the direct measurement of height altogether. The effects of roadway perturbations on the wheels supporting the vehicle are measured and the effective roadway profile is deduced from known characteristics of the wheel and tire. A further attribute of the present method is that the tires of the vehicle provide a "real world" filter for the roadway height variations which eliminates the need for analytical filtering of the resulting measurements, thereby eliminating a source for the introduction of error. Consequently, road surface texture does not contaminate the measurements.
The processing of the measurement data in the prior art methods was based on a digital manipulation in the spatial domain. Although this method provided for vehicle speed independence of the method, the possibility of the introduction of error due to the numerical integration remained. The present invention eliminates that source of error by performing the manipulation of the data in the frequency domain using Fourier Transformations. Further, the present methodology provides for the determination of the spectrum of the roadway perturbations in the frequency domain. This allows for the quantification of amplified perturbation inputs while maintaining the same relative amplitude of input at all frequencies. This methodology allows the testing of vehicles to be accelerated in the sense that the rate of energy input to a vehicle from the roadway is increased by a quantifiable amount while maintaining the relationships of the input at all frequencies.
A high-speed road profiling system was reported by Whittemore in SAE Paper 720094, dated January 1972 in which an attempt was made to determine road profile through the measurement of vertical acceleration and vertical force at the wheel. This prior art used a strain gaged spindle to acquire force data and an analog computer to perform the data analysis. This arrangement failed to accomplish the measurement of forces and data analysis with sufficient accuracy and, as a result of these limitations, practitioners in the field proceeded along the direction of the prior art discussed above. The present invention resolves the problems encountered by Whittemore in that a force transducer of a different construction and sensitivity is incorporated and a digital computation technique is employed to provide the accuracy required.